The diameter of the core, and the refractive index profile of the core and cladding are the basic parameters that specify the transmission characteristics of an optical fiber. Thus, it is very important that these parameters be carefully controlled as the fiber is made. This may involve maintaining them as uniform as possible or, alternatively, may include certain predefined variations. Accordingly, a simple nondestructive technique for accurately observing these parameters is extremely useful. One such technique, which involves measuring the backscattered radiation pattern obtained when the fiber is illuminated by a cw laser beam is described in U.S. Pat. No. 3,879,128. In U.S. Pat. No. 4,161,656, a method of measuring the index profile of optical fibers and fiber preforms using uv illumination is described. More recently, techniques for measuring and controlling the index profile of fiber preforms during fabrication are disclosed in copending application Ser. No. 124,923, filed Feb. 26, 1980, and assigned to applicant's assignee. None of these, however, provide a simple and quick technique for observing the core region of optical fibers and preforms either during or after fabrication.